A ferritic stainless steel sheet is used mostly in the form of press-formed members for kitchen facilities, electric appliances and the like. However, as a ferritic stainless steel sheet is significantly inferior in formability to a SUS304 steel sheet, a typical austenitic stainless steel sheet, it is prone to problems such as cracking in press forming work.
In addition, compared with an ultra-low-carbon steel sheet, a ferritic stainless steel sheet has problems of cracking during press forming caused by its inferior deep drawability and poor shape fixability caused by its higher hardness.
Though there has been strong a need for replacing members of austenitic stainless steel, from the viewpoint of material costs, and members of ultra-low-carbon steel, from the viewpoint of corrosion protection and appearance, with members of ferritic stainless steel, in the event of employing ferritic stainless steel, the poor press formability thereof has been a major obstacle.
In order to solve the problem, various methods have been studied to improve the formability of a ferritic stainless steel sheet and a method wherein the contents of C and N are lowered and elements such as Ti and Nb are added is publicly known as one solution. Despite such improvements, however, formability of a ferritic stainless steel enough to replace a member of austenitic stainless steel or ultra-low-carbon steel with a member of ferritic stainless steel has not been realized.
It is a normal practice in the forming of a stainless steel sheet to apply lubricating oil such as machine oil to the sheet for preventing cracking and die galling during press forming. However, as this requires a cleaning process for removing the lubricating oil after the press forming, there has been a problem of poor operability. In addition, though it is desirable for lubricating oil to have a high viscosity for improving formability, there has been a problem in that the higher the viscosity is, the more often oil remains after cleaning.
As stated above, the problems related to the formability of a ferritic stainless steel sheet have not been solved satisfactorily and, even when the forming thereof is possible, the application and removal of lubricating oil have been required and thus considerable operability has been sacrificed.
As is shown on page 254 of the Press Forming Handbook—In View of Forming Difficulty, the Second Edition, edited by the Thin Steel Sheet Forming Technology Workshop, a lubricated steel sheet having been coated with solid lubricating films beforehand and not requiring lubricating oil has been developed recently. However, simply applying solid lubricating films to a presently existing ferritic stainless steel sheet has been insufficient to realize good formability comparable to that of an austenitic stainless or ultra-low-carbon steel sheet.
As examples of the latest technologies, stainless steel sheets wherein the elongation after fracture and the Lankford values (hereinafter referred to as an r-value) thereof are raised, acrylic or urethane resin is applied to the surfaces, and by so doing improved material properties and the function of lubricating films are combined together, are disclosed in Japanese Unexamined Patent Publication Nos. 2002-60972 and 2002-60973.
By such a method, however, though a limiting drawing ratio (hereinafter referred to as an LDR) measured through cylindrical cup deep-drawing test has been improved, formability has been insufficient for applying the method to those components that require not only deep drawability but also punch stretchability. In addition, by the method, spring back occurs after press forming and thus there has been a problem with shape fixability as well.